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Archived

Avalanche Forecast

Dec 4th, 2016–Dec 5th, 2016
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Mt Hood.

Despite a favorable cooling trend with this storm, westerly winds will build fresh wind slab on lee aspects near and above treeline. Look for wind slab development further downslope than you might expect due to locally strong winds and persistent loading. 

Detailed Forecast

Sunday morning update: 

The below treeline danger has been raised to Considerable because storm slabs should be easily human triggered. Stick to lower angled slopes Sunday. 

Previous discussion: 

A strong frontal system will sweep through Mt. Hood late Saturday night through mid-day Sunday. This system should deliver a nice shot of snow to Mt. Hood with an overall cooling trend. 

Despite a favorable cooling trend with this storm, westerly winds will build fresh wind slab on lee aspects near and above treeline. Look for wind slab development further downslope than you might expect due to locally strong winds and persistent loading. Watch for firm wind transported or hollow sounding snow. 

Storm slab should be a lesser avalanche concern on Sunday. However, in wind protected areas that experience persistent and intense snowfall rates, locally sensitive storm slab may develop.   

In much of the lower part of the below treeline band there is still insufficient snow for avalanches, so watch for early season travel hazards such as terrain traps, rocks and open creeks. 

Snowpack Discussion

Weather and Snowpack

A front crossed the Cascades Friday with around 2 inches of new snow reported at NWAC stations on Mt. Hood through early Saturday morning. The new snow mixed with rain Friday up to at least 5500 feet and a breakable crust was reported on Saturday. Post-frontal showers were generally light on Saturday and moderate west winds continued throughout the day. With the mild snow levels seen so far this season, most avalanche problems have been confined to the most recent storm layers found in the upper snowpack.   

Recent Observations

NWAC pro observer Laura Green was at Mt Hood Meadows ski area on Thursday and reported significant wind transport but only minor small wind slab releases during ski area avalanche control work. 

Laura was touring in Iron Creek Saturday and found a generally stable snowpack with obvious signs of recent wind transport with scoured windward aspects as she traveled near treeline. The Meadows pro-patrol had no significant avalanche control results Saturday. 

Avalanche Problems

Wind Slabs

Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: North, North East, East, South East, South.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1

Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Elevations: Treeline, Below Treeline.

Likelihood: Likely

Expected Size: 1 - 1